By using indexLabelFormatter, you can return any value to be displayed as indexLabel.
Below is the structure of object passed as parameter to the function

e: { // parameter sent to function
chart,
dataSeries,
dataPoint,
index, // Available only in range charts
total, // Available only in stacked, pie and doughnut charts
percent, // Available only in stacked, pie and doughnut charts
}

Note

index parameter is available only in case of range charts. It is undefined in case of other chart types.